1. Field of the Invention
The present invention relates to a hydraulic brake pressure control device for a vehicle, and more particularly, to a hydraulic brake pressure control device for a vehicle comprising an input hydraulic pressure chamber in communication with an output port of a master cylinder, and an output hydraulic pressure chamber in communication with a wheel brake to generate a brake hydraulic pressure corresponding to the hydraulic pressure in the input hydraulic pressure chamber, wherein when the wheel tends to assume a locked state, a volume of the output hydraulic pressure chamber increases in response to introduction, of controlled liquid pressure from anti-lock control means into a control chamber.
2. Description of the Prior Art
In the past, in such a hydraulic brake pressure control device for a vehicle as described above, a piston is actuated in response to hydraulic pressure of the input hydraulic pressure chamber to reduce the volume of the output hydraulic pressure chamber, whereby brake hydraulic pressure corresponding to the input hydraulic pressure chamber is generated from the output hydraulic pressure chamber. During the anti-lock controlling, the piston is actuated in the direction opposite to that previously mentioned by controlled liquid pressure supplied to the control chamber to increase the volume of the output hydraulic pressure chamber.
In such a hydraulic brake pressure control device as described above, a brake hydraulic pressure system is divided into a system from a master cylinder to an input hydraulic pressure chamber and a system from a output hydraulic pressure chamber to a wheel brake, and accordingly, it has been necessary, in supplying working oil to the brake hydraulic pressure system, to fill the working oil into both systems. When the brake is being applied the piston is always operating, and the number of strokes thereof increases, thus deteriorating a durability in one aspect.
In view of the foregoing, the present applicant has proposed, as disclosed in, for example, Japanese Patent Application Laid-Open No. 27749/1986, a hydraulic brake pressure control device for a vehicle in which the brake hydraulic pressure system is unified from a master cylinder to a wheel brake to facilitate filling work of working oil And in order to improve a durability by reducing the number of strokes of the piston, a valve mechanism which is closed in response to movement of a piston during the anti-lock controlling is provided in a partitioning wall between an output hydraulic pressure chamber and an input hydraulic pressure chamber. In this hydraulic brake pressure control device, the aforesaid piston has a first piston defining an input hydraulic pressure chamber and a control chamber and a second piston defining an output hydraulic pressure chamber connected by a piston rod which liquid-tightly and slidably extends through the partitioning wall. According to the hydraulic brake pressure control device as described above, the above-described problems were solved. Generally, the hydraulic brake pressure control device as described above requires that when the wheel is about to assume a locked state during braking, the brake hydraulic pressure is immediately reduced. That is, the responsiveness is good. In order to further improve the responsiveness of the brake pressure control device for a vehicle, as disclosed in the aforementioned publication, it is necessary for the piston to quickly respond when the anti-lock control begins.
In order to respond to such a demand, in the hydraulic brake pressure control device, brake hydraulic pressure of the input hydraulic pressure chamber is exerted as a back pressure on the first piston which receives the controlled liquid pressure of the control chamber. Therefore, a difference in exerting forces applied to the piston by these pressures has to be made as large as possible. It is contemplated that in order to make the exerting force large, a difference between pressure receiving areas of opposite ends of the first piston be increased or the controlled liquid pressure applied to the piston be increased.
In order to increase the difference in pressure receiving areas of the first piston, it is necessary to increase a sectional area of the piston rod since said difference is determined by the sectional area of the piston rod. That is, the diameter of the piston rod need be made large. However, since the valve mechanism is disposed in the partitioning wall, when the diameter of the piston rod is made large, the piston rod interferes with the valve mechanism, and the diameter of the piston rod cannot be made so large. If an attempt is made to forcibly increase the diameter of the piston rod, the valve mechanism has to be provided in a direction further away from the center of the piston rod, and therefore the whole hydraulic brake pressure control device becomes large-sized.
As described above, there is a limit to increase a difference in pressure receiving areas of the opposite ends of the first piston.
On the other hand, in order to increase the controlled liquid pressure, it is necessary to increase the capacity of a hydraulic pressure pump which generates the controlled liquid pressure. However, when the capacity of the hydraulic pressure pump is increased, not only the energy consumption becomes increased but also the whole hydraulic brake pressure control device becomes large-sized.